As shown in FIG. 1, a wireless communication system 10 comprises elements such as a client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station. Cellular wireless communication systems with multiple base stations may be also referred to as wireless communication networks.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station to which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. The terms base station and a cell may be used interchangeably herein. In general, the cells that are in the vicinity of the serving cell are called neighbor cells. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Client terminals used in wireless communication systems may need to search for the network, acquire the network information, camp on to the network and register for service. The aforementioned process is collectively called “network registration.” The network registration process may normally take place in different scenarios that may include but are not limited to powering on the client terminal, attempting to obtain service after a loss of network coverage (e.g., a dropped call due to a “dead spot” in the network), when roaming from one network to another, etc.
The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system is an example of a cellular wireless communication system. In the 3GPP LTE wireless communication system, the air interface is organized into radio frames, subframes, and Orthogonal Frequency Division Multiplexing (OFDM) symbols as shown in FIG. 2, where the radio frame duration is 10 ms, the subframe duration is 1 ms and an OFDM symbol duration is about 70 μs or 85 μs depending on whether Normal Cyclic Prefix (CP) or Extended CP is used respectively. Each radio frame is numbered and identified by the System Frame Number (SFN). The SFN starts from zero and increments for each radio frame up to 1023 and then wraps around to zero and so on. Therefore, the SFN can be represented as a 10-bit number and it is incremented at the start of every radio frame.
The radio frame and subframe boundary are detected by the client terminal during the cell search procedure by first detecting the Primary Synchronization Signal (PSS) and then Secondary Synchronization Signal (SSS) as shown in FIG. 2. The PSS and SSS detection timing is relative to the internal timing of the client terminal and it is referred to herein as timing offset. The radio frame and subframe start timing is derived from the timing offsets of the detected PSS and SSS. The SSS detection requires the PSS time offset as an input from the PSS detection procedure. Therefore, the SSS detection may be scheduled after successful PSS detection. When a client terminal may not be synchronized with any of the base stations, such as in the case of initial power on, it must first find the synchronization information such as the air interface timing and frequency.
In the 3GPP LTE wireless communication system, the network may use a number of different channel bandwidths. However, the synchronization signals PSS, SSS and Physical Broadcast Channel (PBCH) are transmitted in the central 1.4 MHz as illustrated in FIG. 2 where the channel bandwidth used is 3 MHz in this example. Also, the SFN and the Physical Hybrid Automatic Repeat Request (HARM) Indicator Channel (PHICH) configuration information are required for the client terminal to receive further details about the network. The above information is transmitted by each BS in the Physical Broadcast Channel (PBCH). The payload inside the PBCH is referred as Master Information Block (MIB). The MIB is used for further processing in the client terminal for network registration. The PBCH is transmitted in subframe 0 of every radio frame in the central 1.4 MHz bandwidth of the channel as illustrated in FIG. 2.
The timing of the receive window for decoding PBCH is based on the radio frame and subframe timing detected for a given cell based on SSS detection for that cell. Therefore, the PBCH detection may be scheduled only after successful SSS detection for that cell. In the remainder of the present disclosure whenever SSS detection is scheduled it is implicit that it is preceded by a successful PSS detection. Similarly, whenever PBCH detection is scheduled it is implicit that it is preceded by a successful SSS detection.
The payload of the PBCH does not change over a period of four radio frames as shown in FIG. 3. This allows the client terminal to perform combining of the PBCH over four radio frames as shown in FIG. 4. The duration over which the PBCH payload content, i.e., MIB, remains the same is referred herein as MIB Transmission Time Interval (TTI). However, the PBCH contains the SFN in the MIB payload and the change of the SFN in payload occurs every four frames on a boundary where SFN modulo four is equal to zero as shown in FIG. 4. The SFN in the MIB contains only its upper eight most significant bits (MSBs). The two least significant bits (LSBs) of the SFN are zero for the frame where the change of the MIB content occurs. Since the client terminal is not aware of the SFN, the combining must be done over a period of seven frames while pursuing multiple parallel hypotheses as shown in FIG. 5. Each hypothesis starts at a new radio frame and corresponds to the two least significant bits of the SFN being equal to zero. Only one of the four hypotheses can be correct and in the worst case it may be the last hypothesis that may be correct. For the example illustrated in FIG. 5, the hypothesis 4 is correct as the SFN is 104, which has two least significant bits equal to zero. Therefore, the worst case time required for one complete PBCH decode attempt for one cell is seven frames (7*10=70 ms). Furthermore, if the signal conditions are poor, even after exhausting all the hypotheses, the PBCH decoding may fail. The client terminal may have to reattempt to decode the PBCH which may require another 70 ms time to pursue all the hypotheses. Although the SFN portion of the MIB may change from one TTI to another, the remaining portion of the MIB generally remains unchanged over a very long period of time such as hours to days.